1. Field of the Invention
The present invention relates to a production method for a support grid of a nuclear fuel assembly used in, for example, a pressurized water reactor, and a nuclear fuel assembly support grid produced by the production method.
2. Description of the Related Art
In the past, the fuel assemblies used in the nuclear reactor of a light water reactor comprised of arranging a plurality of support grids at prescribed intervals between an upper nozzle and lower nozzle, respectively mounting an instrumentation tube and a plurality of control rod guide tubes on each support grid, upper nozzle and lower nozzle, and holding the fuel rods by inserting them into the grid space of each support grid. Each support grid is composed by crossing thin, band-shaped straps in the form of a grid.
The straps are formed from, for example, zircaloy 2 alloy or zircaloy 4 alloy, and as shown in FIGS. 2A and 2B, each inner strap 2 is assembled in the form of a grid, and the intersections at which each inner strap 2 crosses in a state in which a large number of grid spaces 3 are formed are respectively spot welded in the form of weld P with a laser welding apparatus and so forth. In addition, at the intersections of inner straps 2 and outer straps 4, engaging parts (intersections) are composed by respectively engaging welding tabs 5 formed on both ends of each inner strap 2 with slots 4a of outer straps 4 located on the four sides of support grid 1, and these are then welded in the form of weld R (the state prior to welding is shown in FIG. 2B).
When assembling support grid 1, a material plate made of zircaloy alloy is first punched to a prescribed shape, as shown in the flow chart of FIG. 3. At this time, since concern remains over dimensional stability within the nuclear reactor if support grid 1 is assembled in its original state following punching, each strap 2 and 4 is annealed inside a heat treatment furnace by employing stress relief annealing under conditions that do not reduce strength. Stress relief annealing conditions in this case are, for example, a heating temperature of about 430-500° C. and holding time of about 0.5-4 hours.
Each strap 2 and 4 is then assembled into the form of a grid following stress relief annealing, and welding is performed centering on intersections (including engaging parts) P and R of each strap 2 and 4 to produce a support grid.
However, in this type of support grid, a metal structure welded parts P and R are left welded centering on the intersections of each strap 2 and 4 (welded structure) remains as is in the form of a rapidly cooled structure following welding, and when used by arranging fuel assemblies in high-temperature water inside a nuclear reactor in particular, there is excessive growth of an oxide film on the rapidly cooled structures of welded parts P and R as compared with the ordinary base material serving as the strap parts other than welded parts P and R of each support grid 1.
For example, in the support grid corrosion test shown in FIG. 4, when the changes over time in oxide film thickness were measured for ordinary base material parts and welded parts of the above support grid at a high temperature of 360° C., as shown in the drawing, as the number of days the corrosion test was conducted increased, the oxide films of the ordinary base metal parts and welded parts successively increased. Moreover, the thicknesses of the oxide film according to changes over time of the welded parts exceeded those of the general base material parts in all cases, and the corrosion resistance of the welded parts was lower than that of the ordinary base material parts. As is described in the literature (S. G. MacDonald et al., ASTM STP 754 (1982) 412), the cause of this can be considered to be a loss of added elements of the welded parts due to welding.
Although the corrosion resistance characteristics of the welded parts of such a support grid is well within the allowed range in an ordinary nuclear reactor and do not cause any particular problems, when a nuclear reactor using a high burn-up fuel, such as fuel pellets in which the concentration of U238 within the uranium dioxide is increased to nearly 5%, is operated for a long period of time at high burn-up (high efficiency), although the corrosion resistance of the welded parts may not deviate outside the allowed range, the amount of leeway with respect to the allowed range decreases. In addition, the excessive formation of an oxide film is also not desirable from the standpoint of structural strength of the support grid.